The present disclosure relates to optical scanning devices for use in image forming apparatuses such as digital copiers, laser printers, and laser facsimile machines, and to image forming apparatuses provided with such optical scanning devices.
In general, an optical scanning device is so configured that a light beam emitted from a beam light source device (LD unit) having a laser light-emitting portion (hereinafter “LD”) is directed through a scanning optical system composed of a collimator lens, a cylindrical lens, a deflector such as a polygon mirror, a scanning lens, etc. to be imaged as a beam spot on a scanned surface, while the deflector makes the light beam scan across the scanned surface at a constant speed in the main scanning direction.
There is also provided a synchronization detection sensor (hereinafter “BD sensor”) for detecting the beam deflected by the deflector. Based on the result of detection by the BD sensor, a write clock is varied, thereby to correct the timing with which to start scanning in the main scanning direction with the light beam deflected by the deflector.
In such an optical scanning device, variation in the environmental temperature or the like causes variation in the refractive index of a lens, and hence variation in the characteristics of a scanning lens and the like, and this causes problems such as inconstancy in magnification (in scanning magnification) and wavy vertical lines, leading to degraded image quality. Specifically, due to variation in the refractive index of the scanning lens, a rise in the environmental temperature causes the scanning width to increase, and a drop in the environmental temperature causes the scanning width to decrease. As mentioned above, the scanning of the scanned surface by the optical scanning device is started after the detection of the light beam by the BD sensor. Thus, variation in the scanning width tends to appear at the scanning-end side. This makes variation in the printing position of an image on a recording medium notable.
As a solution, there has been proposed a method of correcting an error in magnification resulting from variation in the environmental temperature. In a known optical scanning device, a write-out detection mirror (SOS mirror) is rotated according to variation in the environmental temperature so as to correct the timing of detection of a light beam by the write-out detection mirror (SOS mirror), thereby to distribute an error in magnification between the scanning-start and scanning-end sides. Specifically, when the temperature inside the cabinet rises, the write-out detection mirror is rotated about its pivot in the direction in which a synchronizing signal is detected earlier so that detection of the light beam by the write-out detection mirror takes place earlier, and thereby printing is started with earlier timing.